Low noise amplifiers (LNAs) are used in many systems where low-level signals must be sensed and amplified. For example, LNAs are utilized in ultrasound imaging equipment to amplify the reflected signal sensed by an ultrasound sensor, and in radio receivers to amplify the radio frequency (RF) signal received by the antenna, The dynamic range of an LNA is limited by the noise floor at the low end and by distortion at the high end. Depending on signal frequency, an LNA can be implemented as an open loop or closed-loop amplifier and may also have a requirement to match a specific source impedance. At frequencies sufficiently below the fT-limit of a semiconductor process, closed-loop, negative feedback designs are possible and offer improved distortion performance. Achieving a wide dynamic range and wide bandwidth with minimal power consumption is a chief design goal.
LNA input and output connections can be single-ended or differential. Where possible, differential output capability improves the dynamic range of an LNA and the circuits that it drives. Providing single-ended to differential signal conversion without an external transformer is especially valuable in many LNA applications.
Single-transistor, open-loop LNA designs are very common for RF frequency applications of 900 MHz and beyond, and often employ “local-series feedback” for some improvement in linearity. Narrowband input impedance matching can be accommodated with passive components, but these amplifiers are not well-suited to the voltage swing requirements and broadband matching needed in many non-RF applications. The prior art circuit shown in FIG. 1 uses a variation of the one-transistor amplifier design for non-RF applications, with an active input impedance matching feedback loop. Despite the excellent noise performance, voltage swing and passive component limitations reduce its usefulness in single-supply designs.
The prior art LNA circuit in FIG. 2 uses a non-inverting operational amplifier 11 configured for improved swing performance, with a conventional differential pair input stage. Input voltage noise is limited by input device thermal noise and the noise of the feedback attenuator resistors. Adequate current consumption in the input pair and an output stage capability for driving loads as small as 40 ohms (shown as R5,R6) result in an input referred voltage noise less than 0.70 nv/rt(Hz). Negative feedback reduces distortion of the input differential pair for frequencies within the bandwidth of the feedback loop. This configuration is not easily generalized to differential outputs or active input impedance synthesis, however.
Optimizing a differential amplifier for noise and distortion performance offers another alternative to LNA design. An example of such a prior art circuit is shown in U.S. Pat. No. 6,118,340, issued Sep. 12, 2000, “Low Noise Differential Input, Differential Output Amplifier and Method,” (the '340 patent). In '340, FIG. 5 shows a feedback-linearized input V-I stage driving well-matched resistive loads. The well-defined gain of this approach allows for accurate active input impedance synthesis, which improves noise figure in applications that require input matching. Differential outputs in this circuit can also be driven from a single-ended input.
This design has various embodiments. When driving a very small gain-setting resistor RS to minimize input noise, high standing currents may be needed in the input stage to accommodate full input and output swing range. When driving a transimpedance load, shown in the '340 patent, FIG. 6, the non-linearity of the output stage is not linearized by the central feedback loop.
The prior art circuit in FIG. 3 shows another differential amplifier approach, from “A Programmable Instrumentation Amplifier for 12-Bit Resolution Systems,” Wurcer, et al., IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 6, December 1982 (the “Wurcer paper”). This circuit provides a unique means of generalizing the non-inverting feedback amplifier approach, described previously, to a fully differential implementation with only one core amplifier stage required, at a considerable savings in complexity and power consumption.
The input signal is applied to bases of a central differential pair, while negative feedback is taken to the emitters. Constant current biasing of the input stage allows each of the input transistors Q1, Q2, Q3, Q4 to complete a negative feedback loop when fed by a resistor attenuator (R57-RG-R56) connected to the differential outputs. This greatly improves input stage linearity for frequencies within the bandwidth of the feedback loop, and offers the possibility of low noise operation if the input standing current and gain setting resistor, RG, are chosen correctly.
The use of operational amplifier buffers for amplifiers A1 and A2 described in the Wurcer paper, which is furthermore referenced to a central bias voltage VB, results in bandwidth limitations and additional complexity and power consumption for use as an LNA stage. Also a single-ended signal applied to only one of the two inputs does not produce a balanced differential output signal.